Initial touch controls implementation

[NyakoFox]

Changes:

This PR adds touch controls to the game. While not being the prettiest UI-wise, it makes it possible to play the game using touch controls, making testing on mobile devices much easier. This PR will be followed up by other PRs which implement more features, such as:

• Menus converted to a mobile-friendly format
• Additional gameplay control styles (Not just D-pad)
• The touch control tutorial
• Saving when you touch a checkpoint
• Level editor controls

Supersedes #1142.

Legal Stuff:

By submitting this pull request, I confirm that...

• My changes may be used in a future commercial release of VVVVVV
• I will be credited in a CONTRIBUTORS file and the "GitHub Friends"
  section of the credits for all of said releases, but will NOT be compensated
  for these changes unless there is a prior written agreement

[NyakoFox]

I've rebased and pushed -- do note that I don't have any apple devices so I'm unable to test for iOS.

[Daaaav]

Went through this whole PR - it looks pretty solid I think! I mostly have some nits here and there.

In addition to the in-context code comments:

• In Touch.cpp, maybe init(void) should be at the top? Maybe get_rect(button, rect) should be right above render(void)? (close to where it's used)
• Maybe the Touch.cpp functions could use comments at the top of the functions to document what the functions mean. Like, touch::get_rect(button, rect) gets a rect from a TouchButton. Then you see touch::get_scale(void) and it gets... the scale? From? A button as well? Is it an API for external use? Oh wait, this one doesn't actually take a button, it's the global graphics scale. (Also, if functions aren't used outside of Touch.cpp then it helps to make them static to make clear that they're local helper functions and not 'API')
• The names of functions could also make things clearer:
  • The get_rect(button, rect) I mentioned above could maybe be clearer as rect_from_button(button, rect)
  • touching_right() sounds a little bit ambiguous - you're not touching the right arrow button, but the right half of the screen
  • The 3 functions touch::button_tapped(id), touch::touching_right(), and touch::screen_down() serve similar purposes (to ask whether something is touched) but all are named completely differently